Bimetal elements



May 16, 1961 Filed Dec. 2, 1957 BIMETAL ELEMENTS lDef/ec/( mu in M33 2 Sheets-Sheet 1 May 16, 1961 A. G. CHERREAU ETAL BIMETAL ELEMENTS Filed Dec. 2, 1.957

2 Sheets-Sheet 2 fig. 2

fig: 5

Ax1o' 0 I 100 200 N/soo 0 Unit d State Pa g'Ihis application is a continuation-impart of our copending application Ser. No. 474,577, filed Dec. 10, 1954, now abandoned, which was already a continuation-in-part application of our original application Ser. No. 174,525, filed July 18, 1950, now abandoned,

This invention relates to bimet al elements.

Bi-metal strips or blades which are used in numerous forms of apparatus having a thermal action such as'thermostats, regulators, circuit breakers and the like comprise, as is known, two alloys: one having-a very high coefficient of expansion and the other having a low c0 efficient of expansion; When the strips,'-suitably as;sembled, are raised to a-certain temperature, the diiference in the expansion of the alloys causes the strip to curve which produces a desired action. This curvature can be expressed by the valueofa specific deflection, namely theamount of displacement of one end of a straight bi metal strip fixed at the other end, the strip having an efiective length of 100 mms. and a thickness of. 1 mm., produced by an increase of temperature of 1 C. The properties of the bimetal strip can also be defined by Villarceaus coefficient:

V= 2(01211-) a1 and a2 being the .coefiicients of expansion of the two componentspt the strip. The components having a high expansion of known bimetal strips are very numerous. Thus, for example, the following metals have been successively used, .viz:

iron, having a coefiicient of expansion atfordin'ary temperaturesofftheorder .of 12.10"; copper, having a coefficient of expansion of the order of 18 to 20.10 nickel, having a coefiicient of expansion of the'order of 16.10";

iron-nickelechromium alloy, having about 23 of nickel; anda low chromium content of 2.5%, the coeflicientof expansion of which'is of the order of to 2210*; an-

other ferro-nickel containing about 15, to 20% of nickel and 5 to 9% of manganese, andhaving a coefiicient of expansion amounting to as much as 22 to23.10

Special alloys have alsobeen recently used containing more than of manganese, the remainder beingnickel and copper in variable proportions; these alloys have the advantage of coefiicients of expansion which can be as In all these cases,rhowever; the comhigh as 27.10-. ponent of low coefiicient' of expansion has been selected from binary iron-nickel alloys.' An alloy fr'equentlyused is a ferro-nickel containing.36% of nickel and -commonly known by the name of Invar (registered trademark owned f' by the applicant company), whose coefi'icient of expansion at room-temperature is the'lowest. This alloy. ex-' hibits, however, an anomaly in its expansion consisting in an extremely rapid increase in the coeflicient of expan sion, starting from 175 C. As the high expansion elementsabove-mentioned or actually known which can be associated with Invar do not show a similar anomaly either with regardtoteniper'ature or to intensity,'' it row Patented May 16, 1961.

lows that the bi-metal strips produced up to the present 7 with Invar lose much of their sensitivity above 175 C.

The invention will be explained by reference to the drawings:

. Figure 1 is a diagram showing the deflection of bi-metal 2 strips as a function of the temperature.

Figs. 2 and 3 are curves taken with a Chevenard diflerential dilatometer showing the relative expansion of an element having a high and an element having a low coetficient of expansion, each with respect to a standard element.

The above elfect is illustrated by curves 1', 2, 3' of Fig. l on which the temperature in degrees centigrade are shown as abscissae and the specific deflections of the bimetal strips in millimetres are shown as ordinates. The

1 three bimetal strips corresponding to the curves 1', 2', 3'

have as the low expansion component a ferro-nickel containing 36% of nickel and as the high expansion compo-, nent either a ferro-nickel chromium alloy containing 23% nickel and 2.5% of chromium (curve 1') or a manganesecopper-nickel alloy containing 74% of manganese, 15% of copper and10% of nickel (curve 2) or pure nickel. (curve 3'). It will be seen that the best of three birnetal strips, namely that producing the curve 2 still showsa decreasein sensitivity above 200 C. Moreover, their :deflectionis linear, i.e. approximately proportional to the increase of temperature, but only between the room temperature and C.'(curve 1'), 150 C. (curve 2') or C. (curve 3) approximately; this temperature limit is still too low for many industrial applications.

The foregoing observations have led the applicants to investigate the possibility of finding means for making bimetal elements with a high and linear deflection up to temperatures which are as high as possible, which is very important for'numero'us practical applications, and particularly'for electric circuit breakers.

In order to find the solution of this problem, it did not sufiice to know the properties of a certain number of metals and alloys, on the one hand with a low coeflicient ofexpansion and on the other hand with a high coeflicient of expansion, and to compare their diagrams of thermal expansion.

As will be seen later, this theoretical study leads to er-' roneou's' conclusions, and only practical work and tests have enabled the object to be attained. Binary iron nickel alloys other than Invar have a comparable anomaly leading to a similar disadvantage in the case of birnetal strips. This anomaly disappears at high temperatures with an increase in the nickel content, but. simultaneously therewith the coefiicient of expansion of the iron-nickel increases very rapidly, which leads to a reduction in the sensitivity of the birnetal strips in substantial proportions.

For these various reasons, the conclusion has been: reached that Invarand the other known iron-nickel binary alloys should not be used for the low expansion component, and a different metal should be sought.

It is known that ternary alloys comprising 16 to 20% of Co and 27 to 30% of Ni--the remainder being substantially 'ironhave'a coeflicient of expansion which although bigger than that of Invar is still very small,

and this within a fairly high temperature range.

It has however been thought up to the present time that the bi-metallic strips in which these alloys are em-f ployed as an element of low coeflicient of expansion could not have a deflection proportional to the temperature between 0 and 400 C. because the thermal expansion of these alloys is irregular in this range of temperature.

This irregularity is shown by a study of their expansion the differential method, which has become'usual since accurate and convenient difierential dilatometerszi have become available, such for example as the wellknown Chevenard instruments, which automatically record the difierence between the expansion of the substance studied and that of a standard having a uniform expansion free from any anomalies, in the vicinity of that 'of the sample under study, and sufliciently creep resistant at the temperatures considered; such standards may be constituted for example by pure molybdenum for the samples with low coefficient of expansion and, for samples with a high ooefiicient of expansion by an alloy of 85 to 88% nickel, 8 to 10% chromium and 2 to 3% tungsten, known by the name of Pyros. V

The diagram of differential expansion reproduced in Fig. 2 relates to an alloy with a high coefiicient of expansion comprising 14.65% copper and 10.81% nickel, the remainder being essentially manganese. The temperature is plotted as abscissae and the differences between the expansion of the sample and that of the standard of Pyros alloy, the well-known law of expansion of which is perfectly regular, are plotted as ordinates; the diagram shows that the expansion of this Mn-Cu--Ni alloy is large and perfectly uniform.

But the expansion diagram of Fig. 3, which shows in a similar manner the difference between the coefficient of expansion of an FeNi--Co alloy with 27.8% nickel and 18.25% cobalt, usable as an alloy with a low coefficient of expansion, referred to that of a standard of molybdenum free from any expansion anomalies, shows on the contrary that the expansion of the Fe-Ni-Co alloy exhibits not only the anomaly corresponding to the point of disappearance of magnetism or Curie point, situated between 400 and 450 C., but also shows between the ambient temperature and 400 a further anomaly of expansion, less pronounced but still perfectly clear; the expansion of the sample is first greater, and then when the temperature rises still further, smaller than that of the standard of molybdenum.

These anomalies of expansion exist for all the FeNi Co alloys, the Curie point of which is situated above 350 C., and in particular for those which contain 27 to 30% of nickel and 16 to 20% of cobalt.

Since the differences in expansion between the two constituents of the bi-metallic strip do not follow a linear law as a function of the temperature, the bi-metallic strip formed by the association of this Fe-Ni--Co alloy of low coeflicient of expansion with an alloy of high 'coefiicient of expansion and free from any expansion anomalies, such as for example the MnCu-Ni alloy which has just been mentioned, should not in theory have a deflection proportional to the temperature, or linear, over the whole range from to 400 C.

Following these scientific indications, in order to form a bi-metallic strip of high sensitivity and with a deflection proportional to the temperature up to 400 C., it would thus be necessary to reject the use of the FeNi Co alloys of low coefficient of expansion of the composition indicated, in spite of their advantages. This is perhaps the reason why it has never been proposed to employ them to constitute the low-expansion element of a bi-metallic strip, associated with an alloy of high coefficient of expansion selected from the group comprising the alloys Mn-Cu-Ni with 13 to 20% of copper and 8 to 12% of nickel, and the FeNi--Mn alloys with about 14 to 20% of nickel and to 9% of manganese which are attractive because of their large and uniform expansion.

It has thus been unexpected and surprising to find, during the course of research tests and check tests that, contrary to what might'have been theoretically expected, the irregularities of the differences between the coefiicients of expansion of the two elements in the temperature range from 0 to 400 C. do not appear in the utilisation of the bi-metallic strip, for reasons which are not yet entirely clear, but which might be connected, at least 4 Thus the applicants have obtained, for the first time, bimetallic strips of high sens'itivity'having a deflection practically proportional to the difference in temperature up to 400 C., and which are entirely satisfactory for all practical requirements, and this constitutes a substantial technical progress: in fact, the specific deflection of these new bi-metallic strips is 1.8 to 2.8 times as great as that of the best bimetallic strips known up to the present time, which have a linear deflection up to 400 C.

In conclusion, the applicants have discovered that the result envisaged in accordance with the invention, that is to say the manufacture of bi-metallic elements of high sensitivity and having a linear deflection up to a temperature reaching, or even exceeding 400 C., has been obtained by the combination of two elements constituted respectively by the alloys specified below.

Alloy with a small coefficient of expansion:

These three alloys may, of course, include in addition the usual contents of secondary elements, such as silicon (or manganese in the case of the first) etc., of impurities such as sulphur, phosphorus, etc.

The bi-metallic members obtained by the association of these alloys are free from anomalies likely to hinder their operation or their use, up to temperatures of the order of 400 C.

The element with a high coeflicient of expansion has preferably a composition which falls within the following limits:

Copper approximately 15%.

Nickel approximately 10%.

Manganese Substantially thejremainder.

Nickel approximately 15%.

Manganese approximately 7%.

Iron Substantially the remainder.

The same reservations apply as in the previous case with respect to the presence of secondary elements or impurities.

The electrical resistivities of the bimetallic strips in accordance with the invention may vary within -fairly wide limits, as shown by the examples below:

(1) The bi-metallic strip, of which the low-expansion element is an Fe-Ni-Co alloy of 27% nickel and 18% cobalt, and the high-expansion element is an Fe-Ni-Mn alloy "with 15% nickel and 7% manganese has 'aspecific deflection of 0.115 mm. per degree centigrade, and a resistivity of 50 microhms per sq. cm. per cm.

Curve 1 on the Fig. 1 shows that the deflection of such a bi-rnetallic'strip having a length of 10 cm. and a thickness of 1 mm., is practically proportional to the temperature up to above 400 C.

(2) The bi-metallic strip, of which the low-expansion element is an FeNi-'Co alloy with 27% nickel and 18% cobalt, and the high-expansion element is an Mn=Cu-Ni alloy with 15% copper and 10% nickel has a specific deflection of 0.170 mm. per degree centigrade and a'resistivity of 70microhms per-sq. cm. per cm.

. Curve 2 on the .Fig. 1 shows that the :deflection of no part, to phenomena of relaxationor accommodation. M such a bi-metallic strip having a length of 10 cm. .and-

aesaooa a thickness of 1 mm., is practically proportional to the temperature up to 400 C.

The deflection of these bi-metallic strips is much greater than that of the bi-metallic strip of which the lowexpansion element would be of an Fe-Ni-Co alloy of the same composition as in these two examples, and the high-expansion element were of pure nickel.

Without departing from the scope of the invention, known means may be employed for modifying the electrical resistance of the bi-metallic strips described, for example by interposing between the two basic elements a sheet of an alloy of high or low resistivity; for example, in the second case, a sheet of nickel, or of iron, or of brass, or a coating of brazing-solder, etc.

What we claim is:

1. A bimetal element of high sensitivity, capable of linear deflection with reference to a temperature between 0 and 400 0., comprising a first elongated part having a low coeflicient of expansion and formed from an alloy consisting of iron, nickel and cobalt, the alloy comprising 27 to 30% of nickel and 16 to 20% of cobalt, the remainder being substantially iron, a second elongated part longitudinally and closely united to the first part and made of an alloy having a high coefficient of expansion selected from the group consisting of manganesecopper-nickel alloys with 13 to 20% of copper and 8 to 12% of nickel, the remainder being substantially manganese, and iron-nickel-manganese alloys comprising 14 to 20% of nickel and 5 to 9% of manganese the remainder being substantially iron.

2. A bimetal element according to claim 1, wherein the part having a high coefiicient of expansion is of manganese-nickel-copper alloy comprising approximately of manganese, 15% of copper and 10% of nickel.

3. A bimetal element according to claim 1, wherein the part having a high coefiicient of expansion is of ironnickel-manganese alloy comprising approximately 15% of nickel and 7% of manganese.

References Cited in the file of this patent UNITED STATES PATENTS 1,843,903 Scott Feb. 2, 1932 1,902,589 Scott Mar. 21, 1933 1,939,085 Scott Dec. 12, 1933 1,987,714 Scott Jan. 15, 1935 2,062,335 Scott Dec. 1, 1936 2,062,836 Scott Dec. 1, 1936 2,234,748 Dean Mar. 11, 1941 2,349,577 Dean May 23, 1944 2,403,895 Alban July 16, 1946 

1. A BIMETAL ELEMENT OF HIGH SENSITIVITY, CAPABLE OF LINEAR DEFLECTION WITH REFERENCE TO A TEMPERATURE BETWEEN 0 AND 400*C., COMPRISING A FIRST ELONGATED PART HAVING A LOW COEFFICIENT OF EXPANSION AND FORMED FROM AN ALLOY CONSISTING OF IRON, NICKEL AND COBALT, THE ALLOY COMPRISING 27 TO 30% OF NICKEL AND 16 TO 20% OF COBALT, THE REMAINDER BEING SUBSTANTIALLY IRON, A SECOND ELONGATED PART LONGITUDINALLY AND CLOSELY UNITED TO THE FIRST PART AND MADE OF AN ALLOY HAVING A HIGH COEFFICIENT OF EXPANSION SELECTED FROM THE GROUP CONSISTING OF MANGANESECOPPER-NICKEL ALLOYS WITH 13 TO 20% OF COPPER AND 8 TO 12% OF NICKEL, THE REMAINDER BEING SUBSTANTIALLY MANGANESE, AND IRON-NICKEL-MANGANESE ALLOYS COMPRISING 14 TO 20% OF NICKEL AND 5 TO 9% OF MANGANESE THE REMAINDER BEING SUBSTANTIALLY IRON. 